In recent years, in photolithography technology, with the trend toward a higher degree of integration and a higher function of an integrated circuit, the refinement of the integrated circuit is advancing. The exposure tool is hence required to form a circuit pattern image with high resolution on a wafer surface at a long focal depth, and shortening of the wavelength of an exposure light source is being advanced. The exposure light source is further advancing from conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) and a KrF excimer laser (wavelength: 248 nm), and an ArF excimer laser (wavelength: 193 nm) is coming to be employed. Also, in order to cope with a next-generation integrated circuit whose circuit pattern line width will become 100 nm or less, use of an F2 laser (wavelength: 157 nm) as an exposure light source is regarded as being leading. However, it is considered that even this would be able to cover only the generation with a line width of up to 45 nm.
Under these circumstances, a lithographic technique employing typically a light having a wavelength of 13 nm among EUV lights (extreme ultraviolet light) as the exposure light source is considered to be applicable over the plurality of generations of 45 nm or finer and has attracted attention. The principle of image formation of EUVL is identical with that of the conventional lithography from the viewpoint that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting light therethrough in the EUV light energy region, a refractive optical system cannot be used. Accordingly, the optical systems are all a reflecting optical system.
The optical member of an exposure tool to be used for EUVL is basically configured with (1) a substrate, (2) a reflective multilayer formed on the substrate and (3) an absorber layer formed on the reflective multilayer. The optical member for an exposure tool to be used for EUVL is a reflection type, and thus, the substrate is not necessarily required to have translucency. However, an extremely low thermal expansion material having transparency has been desired so as to make evaluation or inspection possible for the purpose of evaluating the homogeneity or the surface smoothness by using an interferometer, etc. so that the substrate will not deform even when irradiated with EUV light, or for the purpose of determining the presence or absence of internal defects such as bubbles or striae by microscopic or visual inspection.
Also, a transparent low thermal expansion material is widely used for various materials which are strictly required to have low thermal expansion properties and transparency, for example, optical member materials, materials for a ring laser gyroscope, precision member materials such as standards for precision measurement, various electronic materials and the like.
The extremely low expansion material having transparency includes a TiO2-containing silica glass represented by ULE #7972 (trade name) manufactured by Corning Incorporated and a transparent crystallized glass represented by ZERODUR (trade name) manufactured by SCHOTT AG.
There is a U.S. patent application which discloses a method including forming a TiO2—SiO2 porous glass body, converting it to a glass body and then obtaining a mask substrate (see, for example, Patent Document 1).
The TiO2—SiO2 glass is known as an extremely low thermal expansion material having a coefficient of thermal expansion lower than that of silica glass. Also, since the coefficient of thermal expansion can be controlled by the TiO2 content in the glass, a zero-expansion glass whose coefficient of thermal expansion is close to 0 can be obtained. Accordingly, the TiO2—SiO2 glass involves a possibility as a material to be used in an optical member of an exposure tool for EUVL. However, since it contains a large amount of an OH group, there are absorptions at several wavelengths, e.g., near 2,700 nm. Furthermore, since it contains Ti3+, there is an absorption at a wavelength in the visible region.
On the other hand, a crystallized glass is composed of a crystalline phase exhibiting negative thermal expansion and a glass phase exhibiting positive thermal expansion, and can be a zero-expansion material having a coefficient of thermal expansion of close to zero by controlling a heat step for crystallization. Also, since the crystal grain is small and since the difference in refractive index between the crystalline phase and the glass phase as a matrix is small, it becomes transparent. Accordingly, there is a possibility to obtain a material having excellent thermal expansion characteristics by contriving the composition of a mother glass or a heat treatment step. However, since the change in dimension relative to a change in temperature exhibits hysteresis due to structural relaxation, the crystallized glass has a problem in absolute dimensional accuracy. Furthermore, since the crystallized glass has absorption in the visible region, it was not suitable for the evaluations of homogeneity, surface smoothness and internal defects, which necessitate high transparency in the visible region. Furthermore, an optical member of an exposure tool to be used for EUVL is required to have an extremely smooth surface, e.g., a surface roughness Ra of 0.15 nm or less. However, there was a problem that a smooth surface is hardly obtained due to influences of the crystal grains.
As the extremely low thermal expansion material, there is a silica glass containing Sn and Ti (hereinafter referred to as “SnO2—TiO2—SiO2 glass”). It has been disclosed a method including forming an SnO2—TiO2—SiO2 porous glass body, converting it to a glass body and then obtaining a mask substrate (see, for example, Patent Document 2).
The SnO2—TiO2—SiO2 glass is known as an extremely low thermal expansion material having a coefficient of thermal expansion lower than that of silica glass. Also, since the coefficient of thermal expansion can be controlled by the SnO2 content or the TiO2 content in the glass, a zero-expansion glass whose coefficient of thermal expansion is close to 0 can be obtained.
Accordingly, the SnO2—TiO2—SiO2 glass involves a possibility as a material to be used in an optical member of an exposure tool for EUVL. However, since the SnO2—TiO2—SiO2 glass disclosed in Patent Document 2 has SnO2 concentration distribution in the glass, it involves a local fluctuation of coefficient of thermal expansion and a local fluctuation of refractive index. Furthermore, since an SnO2 crystal is easily deposited due to influences of the SnO2 concentration distribution, there were problems that a thorough temperature control must be carried out in order to prevent the transmittance from lowering and obtain a smooth surface.
Citation List
Patent Literature
Patent Document 1: US-A-2002/157421
Patent Document 2: JP-A-2007-238425